This invention relates to optical cables and, more particularly, to the design of non-metallic strength rods used therein.
Optical fibers are in widespread use today as transmission media because of their large bandwidth capabilities and small size. However, optical fibers are mechanically fragile, exhibiting low strain fracture under tensile loading and degraded light transmission when bent. Transmission degradation, which results from bending, is referred to as macrobending and microbending losses. Consequently, cable structures have been developed to protect the optical fibers in various situations. Additionally, optical cables use glass fibers as a communications medium rather than copper wires; and while glass fibers are relatively strong, care must be taken to avoid excessive tensile stress because they are quite thin and are not ductile. Moreover, the optical transmission characteristics (e.g., index of refraction) of glass change in response to the application of stress. Therefore, strength members are generally included in optical cables to receive most or all of the stress due to tensile loading before it can be transferred to the optical fibers.
An optical cable having excellent strength performance is described in U.S. Pat. No. 4,844,575 that issued to Kinard et al. on Jul. 4, 1989. This cable comprises one or more optical fibers that are disposed within a cylindrical plastic tube, and a pair of metallic rods that are positioned on diametrically opposite sides of the tube and extend along the length of the cable. Steel is a preferred strength member for an optical cable because its tensile stiffness is suitable for receiving axial loading and its compressive stiffness is suitable for inhibiting contraction of the cable. Moreover, the cross-section area of a steel strength member is relatively small in comparison with other materials so that it does not undesirably increase the overall diameter of the optical cable. Nevertheless, there has been a long felt need for an all-dielectric cable construction. Such a cable could be strung from building ducts to service distribution points, and would obviate the need for grounding connections at splice points that add to the cost of cable installations. Further, such a dielectric cable would decrease the probability of lightning strikes.
A dielectric optical cable having excellent strength performance is disclosed in U.S. Pat. No. 5,109,457 that issued to Panuska et al. on Apr. 28, 1992. In this patent, the metallic rods of the Kinard et al. patent are replaced with non-metallic rods for tensile and compressive stiffness, and non-metallic rovings for added tensile stiffness. The rods are made from E-glass fiber filaments that have been impregnated with epoxy, and the rovings are made from E-glass fiber filaments without epoxy. FIGS. 5 and 6, herein, show this all-dielectric optical cable in greater detail. This combination of rods and rovings provides excellent strength and flexibility in a relatively small-diameter cable; however, it is desirable to minimize the number of components in a strength member system to simplify its manufacture. Merely increasing the diameter of the glass rods to eliminate the rovings unduly increases the overall diameter of the cable.
A dielectric strength member system constructed entirely of filamentary strands, e.g., yarn, with superior tensile modulus is desired in order to minimize the overall cross section area of the strength members and hence, the completed cable. Alternatively, a strength member system consisting of high tensile modulus filaments within an epoxy matrix is desired for improved compressive properties as well as minimal cross-section area. A composite strength member, consisting of aramid fibers within an epoxy matrix is commercially available from NEPTCO, for example, under the name ARALINE(copyright). Such strength members are known to possess tensile moduli that are superior to glass strength member of equal cross-section area. However, they are also known to have compressive properties such as modulus and strength) that may be insufficient to adequately isolate the optical fibers in the completed cable from stresses or strains imposed on the cable during manufacture, installation, or subsequent handling after installation.
Dielectric crossply sheaths having one or more layers of helically disposed rods and rovings are also well known. An example of a cable having such a strength system is disclosed in U.S. Pat. No. 4,743,085. This construction typically provides for the most flexible and compact structure, but it is relatively expensive to manufacture such a cable due to the increased number of elements, and its kink resistance is relatively low due to reduced strength member size and jacket thickness.
Accordingly, it is desirable to provide a dielectric strength system for an optical cable that minimizes strength member cross-section area while still providing suitable compressive and tensile properties.
A hybrid strength member for an optical cable is made from all-dielectric materials, and provides excellent compressive and tensile properties within a single structure. Each strength member (also referred to as a rod) includes two concentric layers of filamentary strands that are embedded in a thermoset material. One layer primarily comprises filamentary strands whose tensile modulus exceeds 12 million pounds per square inch (psi) for tensile strength, while the other layer primarily comprises glass and/or ceramic fibers for compressive strength. The dielectric rods have compressive properties that are effective to inhibit substantial contraction of the cable during thermal cycling and to withstand compressive loads imposed on the cable during installation and handling. The dielectric rods also have a tensile modulus that is effective to receive tensile loads without substantial transfer of same to the glass fibers.
In an illustrative embodiment of the invention, two layers (inner and outer) of filamentary strands are used to construct the hybrid strength member. The inner layer of filamentary strands comprises packages of aramid fibers while the outer layer comprises packages of glass fibers. These packages are coated with heated epoxy and then passed through a die for consolidation and removal of excess epoxy. The resulting rod is then cured via multiple cooling stages. During passage through the die and subsequent cooling, a differential tension is maintained between the aramid and glass fibers to compensate for differential thermal expansion effects.
The dielectric rod is intended for use in an optical cable where it is at least partially embedded in a plastic jacket of the cable to receive compressive and tensile loading. To that end, the dielectric rod may be coated with a material to enhance coupling, without adhesion, between the rod and the plastic jacket. The coating is selected to be chemically different from the jacket material in order to prevent adhesion. Further, the material should be relatively soft to provide a high coefficient of friction with the jacket. Illustratively, the coating material has a hardness that is less than 80 D on the Shore durometer scale and comprises a thermoplastic elastomer, a crosslinkable rubber or a hot melt composition.
In the illustrative embodiment of the invention, dielectric rods are positioned on diametrically opposite sides of a tubular member (core tube) that includes a number of optical fibers. Optionally, a waterblocking filling material is used to fill any voids within the core tube. In a preferred embodiment of the invention, the dielectric rods are linearly positioned within the cable and are substantially parallel to its longitudinal axis. Nevertheless, smaller-diameter dielectric rods may be installed in the cable with helical stranding or with a reverse-oscillating lay (also known as an xe2x80x9cS-Zxe2x80x9d twist).